Why useReducer doesn't update state in React context

I'm using useReducer and context to update my state in App.js then send data to the database but it doesn't update the state and always is null.

App.js

import AuthContext from './context';
import screenA from './screenA';

export default function App() {

 const initialLoginState = {
   email: null,
 };
const loginReducer = (prevState, action) => {
   switch (action.type) {
     case 'Email':
       return {
         ...prevState,
         email: action.Email,
       };
   }
 };

const authContext = React.useMemo(
   () => ({
     email: emailUser => {
       dispatch({type: 'Email', Email: emailUser});
       console.log(loginState.email);
     },
     signIn: async () => {
       try {
         fetch('FakeApi', {
           method: 'POST',
           headers: {
             Accept: 'application/json',
             'Content-Type': 'application/json',
           },
           body: JSON.stringify({
             email: loginState.email,
             date: '2021-9-20',
           }),
         });
       } catch (e) {
         console.log(e);
       }
     },
   }),
   [],
 );
 const [loginState, dispatch] = React.useReducer(
   loginReducer,
   initialLoginState,
 );

return (
   <AuthContext.Provider value={authContext}>
         <screenA />
   </AuthContext.Provider>

Blockquote context

I create a separate context component

context.js

import React from 'react';

export const AuthContext = React.createContext();

Blockquote screenA

In screenA I use email and signIn to send data to App.js and then save this data in the database screenA.js

import {AuthContext} from './context';

function screenA() {
 const {email,signIn} = React.useContext(AuthContext);
 return (
   <View style={{marginTop: 150}}>
     {/* Count: {state.count} */}
     <Button
       title="sent email"
       onPress={() => {
         email('example@gmail.com');
       }}
     />
     <Button
       title="signIn"
       onPress={() => {
         signIn();
       }}
     />
   </View>
 );
}

export default screenA;

The state is being updated, the problem is you are trying to write the value to the console right after "dispatch", at that point in time the state wasn't updated yet. If you want to view the updated value, add this useEffect to App.js

  useEffect(() => {
    console.log(loginState.email);
  }, [loginState.email]);

This useEffect will be triggered when "loginState.email" change.

You should consider using React Developer Tools to validate state changes.